Moisture Sensitive Auxetic Material

ABSTRACT

An auxetic fiber and corresponding material that not only responds to an external force, but also responds to moisture. The auxetic fiber is made in part from a moisture activated shrinking filament. Even if no external force is applied to the fiber, a pseudo tensile force is created by wetting the auxetic fiber.

This application is a continuation of application Ser. No. 12/337,821filed on Dec. 18, 2008. The entirety of application Ser. No. 12/337,821is hereby incorporated by reference.

BACKGROUND

This invention relates to auxetic materials, in particular, to amaterial comprising an array of interconnected moisture sensitiveauxetic fibers.

Auxetic materials are materials that have a negative or effectivelynegative Poisson's ratio. In contrast to most conventional materials,auxetic materials possess the property that under a tensile load thematerial expands perpendicularly to the axis along which the tensileload is applied. In other words, auxetic materials expand as they arestretched. Conversely, materials are also auxetic if a compressive loadapplied along an axis results in a reduction in the dimension of thematerial along an axis perpendicular to the axis along which thecompressive load is applied. Most materials exhibit a positive Poisson'sratio, this ratio being defined by the ratio of the contractiletransverse strain relative to the tensile longitudinal strain.

Prior art auxetic materials are only activated by an applied externalforce and can essentially be divided into two categories. One categorycomprises honeycomb like polymeric materials, and the other categorycomprises materials formed by particles linked by fibrils. However, bothof these categories of auxetic materials have significant drawbackspreventing commercialization on an industrial scale. In particular,there are problems in producing such auxetic materials reliably andcost-effectively using techniques which are suitable forcommercialization. Recently in order to overcome above problems, ahelical fiber was developed. However, the structural characteristics ofall prior art auxetic materials made them unsuitable for use in devicesor articles that are not exposed to an external force. Thus, what isneeded is an auxectic material that not only uses a tensile force tocreate an auxetic effect and/or pores in the material, but uses a secondcatalyst for creating an auxetic effect and/or pores in the material.

SUMMARY

One aspect of the present invention is an auxetic fiber including afirst component in the form of a moisture sensitive shrinkable filament,and a second component including an elastomeric material, wherein thefirst component is wrapped about the second component in a helicalconfiguration. The first component is made from one or more of thefollowing materials: modified cellulose fibers, rayon, cotton,carboxymethylated cotton, methylated cotton, ethylated cotton,hydroxyethylated cotton, sulfated cotton, sulfonated cotton, phosphatecotton, cationic cotton, amphoteric cotton, sodium acrylate-, acrylicacid-, acryInitrile- or acrylamides grafted cellose fiber andcrosslinked fibers thereof, modified wool, modified silk, modifiedsynthetic fiber, a saponified acylonitrile series of fiber, and vinilonfiber partially esterfied by maleic acid, and yarns made from one ormore of the foregoing fibers.

In another aspect of the present invention there is an array of auxeticfibers including a first and a second auxetic fiber each having a firstcomponent, wherein the first component includes a moisture activatedshrinkable filament, and an elastomeric second component, and whereinthe first component is wrapped about the second component in a helicalconfiguration. The second component is made from one or more of thefollowing materials: silicone rubber, natural rubber, poly(urethane) andits derivatives, natural rubber, polyisoprene, bytyl rubber and itsderivatives, polybutadiene, styrene-butadiene rubber, chloroprenerubber, polychloroprene, neoprene, baypren, ethylene propylene rubber,ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylicrubber, fluorosilicone rubber, fluoroelastomers, perfluoroelastomers,polyether block amides, chlorosulfonated polyethylene, ethylene-vinylacetate, nitrile rubber or polyamides, and nylon.

The present invention will now be described in detail with reference toembodiments illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation of a single auxetic fiber of the presentinvention shown in a dry, unstressed state.

FIG. 2 is a front elevation of the fiber shown in FIG. 1 except thisfiber is in a wetted state.

FIG. 3 is a plan view of a material made from the fibers of FIG. 1 witheach fiber having a different handedness with respect to adjacentfibers.

FIG. 4 is a plan view of the material shown in FIG. 3 in a wetted state.

FIG. 5 a is the material of FIG. 3, wherein a tensile force has beenapplied in the X-direction.

FIG. 5 b is the material of FIG. 5 b, wherein an additional tensileforce has been applied by wetting.

FIG. 6 a is an array of fibers such as that shown in FIG. 1, the arrayin a wetted state to create pores.

FIG. 6 b is an array of fibers such as that shown in FIG. 6 with atensile force applied in an X-direction to create even larger pores.

DETAILED DESCRIPTION

FIG. 1 shows one exemplary embodiment of auxetic fiber 10 in a dry,unstressed state. The fiber 10 may be utilized in the production ofporous or non-porous materials according to the invention. The auxeticfiber 10 comprises a first component 12 and a second component 14. Thefirst component 12 may be wrapped around the periphery of the secondcomponent 14 forming a helix. Desirably, as shown in FIG. 1, thewrapping of the first component 12 around the second component 14 causesno deformation of the second component 14 so that it has a generallylinear configuration. The first component is desirably formed from amoisture-sensitive material having a relatively high modulus ofelasticity. The second component is preferably formed from a material oflower modulus of elasticity than the first component.

The first component 12 may be a moisture sensitive filament that shrinks(e.g. becomes shorter) when wetted with water, urine, or otherwater-based liquids. Suitable materials for the first component 12 areliquid shrinkable filaments made from film, fiber, threads, foamedbodies, or the like. Those materials capable of shrinking by 10% ormore, or particularly 20% or more when exposed to an aqueous liquid aredesirable. Materials such as this include modified cellulose fibers(e.g. cotton and rayon) such as carboxymethylated cotton methylatedcotton, ethylated cotton, hydroxyethylated cotton, sulfated cotton,sulfonated cotton, phosphate cotton, cationic cotton, amphoteric cotton,sodium acrylate-, acrylic acid-, acryInitrile- or acrylamides graftedcellose fiber and crosslinked fiber thereof; wool or silk modified inthe same manner as above; modified synthetic fiber, such as partiallysaponified acylonitrile series of fiber and vinilon fiber which ispartially esterfied by maleic acid; and yarns made from these fibers. Adesirable material for the first component is a yarn or filamentavailable from Nitivy Company, Japan (SOLVRON Yarn—SHC Grade). Thiswater shrinkable component is a polyvinyl alcohol filament.

Suitable materials for the second component 14 include siloxane,silicone rubber, natural rubber, poly(urethane) and its derivatives,natural rubber, polyisoprene, bytyl rubber and its derivatives,polybutadiene, styrene-butadiene rubber, chloroprene rubber,polychloroprene, neoprene, baypren, ethylene propylene rubber, ethylenepropylene diene rubber, epichlorohydrin rubber, polyacrylic rubber,fluorosilicone rubber, fluoroelastomers, perfluoroelastomers, polyetherblock amides, chlorosulfonated polyethylene, ethylene-vinyl acetate,nitrile rubber or polyamides such as nylon and the like. The material ispreferably capable of deformation without fracture. The second componentmay be an elastic fiber, rod or hollow tube, particularly consisting ofa material with an intermediate or a low modulus of elasticity. Itshould be noted that it is possible for the second component 14 to beformed from a material of relatively high modulus of elasticity, such asnylon, provided it is used in combination with a first component formedfrom a material of higher modulus of elasticity.

The first component 12 and/or second component 14 may be formed from acontinuous material. Preferably, the first component 12 and/or secondcomponent 14 are elongate. The first component 12 and/or secondcomponent 14 may be at least a hundred times as long as their maximumcross-sectional dimension.

Advantageously, at least one of the first component 12 and the secondcomponent 14 is helically wrapped around the other component. Thewrapping of the first component 12 around the second component 14 may bein the form of a helix that may have a constant pitch along the secondcomponent 14. The pitch may be between zero degrees and ninety degreesrelative to the axis.

It will become apparent to the skilled reader that, with a given auxeticfiber of this type, at some applied strains both the first and secondcomponents may be helically wrapped around the other component, but atother (higher) applied strains the first component may substantiallystraighten so as to no longer be helically wrapped around the secondcomponent. In principle, the second component might be substantiallystraight at low applied strain.

The first component may have a diameter that is between 0.01 and 1 timesthe diameter of the second component. The first component may have across-sectional area that is between 0.001 and 1 times thecross-sectional area of the second component.

FIG. 2 shows the fiber 10 in a wetted state. It is observed that thefirst component 12 shrinks when exposed to moisture, thereby pulling thesecond component 14 into a deformed configuration and creating pores.The shrinking first component 12 acts as a pseudo-tensile force that isapplied without having to stretch the material. It is noted here thatpores may not always be desired. Thus, the material when wetted may onlydemonstrate a negative Poisson's ratio without pores.

FIG. 3 shows a set of auxetic fibers 10 of the type described above withreference to FIG. 1. In particular, it should be noted that the helicesformed by the first components 12 a, 12 b of the auxetic fibers 10 a, 10b are in phase and of opposite handedness. It is contemplated that theset of fibers could have the same handedness (not shown).

The configuration shown in FIG. 3 depicts the pair of adjacent auxeticfibers in a dry state at a zero tensile load. FIG. 4 depicts the samepair of auxetic fibers 10 a, 10 b at a loaded state. This load is eitherdue to an external tensile force and/or caused by the wetting andshrinking of the first component 12 a, 12 b. It can be seen that theresult of applying a relatively high tensile load (either by wettingand/or stretching) along the x axis, i.e., along the length of theauxetic fibers, is that the first components 12 a, 12 b straighten. As aresult of the straightening of the first components 12 a, 12 b, thediameter of the helices formed by the second components 14 a, 14 bincreases. In particular, in the regions 22 in which the pores 20 may bedefined, one second component 14 a is positively displaced in they-direction, whereas the other second component 14 b is negativelydisplaced in the y-direction. The effect of the displacements caused bythe application of a tensile load is to increase the thickness of thematerial and possibly to create pores 20.

The present invention may utilize this phenomenon to control pore sizein porous materials fabricated from auxetic fibers.

Referring still to FIG. 4, when the first components 12 a, 12 b arewetted, the second components 14 a, 14 b are deformed. Owing to thehelical configuration imposed upon the second components 14 a, 14 b bythe first components 12 a, 12 b, a similar relationship exists betweenthe second components 14 a, 14 b, i.e. the helices formed by the secondcomponents 14 a, 14 b are in phase but are of opposite handedness. As aresult of this configuration, pores may be formed in regions such asthat marked 22, where the helices formed by the second components 14 a,14 b are oppositely opposed to the maximum extent.

FIGS. 5 a and 5 b depict a porous material 40. The porous material 40comprises an array of pairs of adjacent auxetic fibers of the type shownin FIGS. 3 and 4. The array of pairs of auxetic yarns forms a flat sheetfabric. The fabric shown in FIGS. 5 a and 5 b is a woven fabric, and forreasons of presentational simplicity, these figures depict only the warpfibers. In other words, the weft threads are not shown in FIGS. 5 a and5 b, although the skilled reader will appreciate that weft threads willbe present in the woven fabric so as to interconnect warp fibers. Theweft fibers may be auxetic or non-auxetic in nature. FIG. 5 a depictsone embodiment of a porous material 40 with a zero tensile load appliedalong the X-axis. It can be seen that the pores 20 are essentiallyclosed under such conditions. FIG. 5 b depicts the porous material 40having a relatively large tensile load is applied along the x axiscausing the fibers 10 to straighten. It is seen that the application ofthe tensile load causes the pores 20 to open. The load may be causedonly by wetting, and/or by an external tensile load.

It is contemplated that the second component 14 could be wrapped aroundthe first component. It is further contemplated that the secondcomponent may be in the form of a helix wrapped about a straight firstcomponent 12. Also, the pitch of the first component 12 helix may be thesame as the pitch of the second component 14 helix.

The auxetic fibers and the material made therefrom have a negativePoisson's ratio. Fibers and/or materials having a Poisson's ratio ofbetween 0 and 35 are preferred. The Poisson's ratio of a fiber is inpart dependent on the pitch of the first component. A steep pitch givesrise to a relatively low auxetic effect over a relatively large strainrange whereas a shallow pitch gives rise to a relatively large auxeticeffect over a relatively narrow strain range.

FIG. 6 a shows a filter material 60 for face masks made from auxeticfibers and defining a plurality of pores 62. FIG. 6 a depicts the filter60 under conditions of low tensile load in the x direction. Under suchconditions, the pores 62 are closed or at least of relatively smalldimension. FIG. 6 b shows the filter 60 under conditions of high tensileload applied along the x axis. Under such conditions of high tensileload, the dimensions of the pores 62 increase, i.e. the pores open up.The tensile load is caused by applying an external force and/or bywetting with an aqueous solution. Thus, by stretching or wetting afilter constructed from appropriately sized auxetic yarns, it ispossible to control the pore size. It is contemplated that someapplications require an auxetic material that does not develop pores,but simply has a negative Poisson's ratio.

By varying the applied tensile load to the filter material, it ispossible to utilize a single filter which is capable of providing avariety of pore sizes. Thus, a single filter can be used in a number ofdifferent applications. The invention is not limited to the embodimentsand examples provided above. Rather, a wide range of applications can beenvisaged in which the pore size of a porous material of the typeprovided by the invention is controlled for an advantageous purpose. Forexample, breathable fabrics might be provided having a plurality ofpores which open up when a wearer of the fabric undertakes an energeticactivity that causes moisture to be released in the form of sweat,thereby causing the pores 62 to open.

The fiber material of the present invention could be used not only forface masks, but for garments where it is desirable to control humidity.Such garments include diapers, incontinence pants, or protective suits.Furthermore, the material could be used as a component in water diapers(e.g. HUGGIES® LITTLE SWIMMERS® Swim Pants) to enhance water drainage.Yet another use includes the release of actives as the pores enlarge.For example, if an auxetic fiber is used as a dental floss that containsa healthy ingredient for gums, the floss will release that ingredientduring use because it is activated by saliva.

In a further embodiment, the auxetic fibers could be placed into anarray that is used for wetness detection. If the auxetic material islayered over an indicator material having a contrasting color or shade,the indicator material will be seen through the auxetic material once ithas been wetted and pores have then formed. This application may beuseful for wetness detection in a diaper or training pant. Many otheruses are possible.

While particular embodiments of the present invention have beenillustrated or described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. Further, it shouldbe apparent that all combinations of such embodiments and features arepossible and can result in preferred executions of the invention.Therefore, the appended claims are intended to cover all such changesand modifications that are within the scope of the invention.

1. An auxetic fiber comprising: a first component comprising a moisturesensitive shrinkable filament; a second component comprising anelastomeric material, wherein the first component is wrapped about thesecond component in a helical configuration, and wherein the firstcomponent is selected from the group consisting of: modified cellulosefibers, rayon, cotton, carboxymethylated cotton, methylated cotton,ethylated cotton, hydroxyethylated cotton, sulfated cotton, sulfonatedcotton, phosphate cotton, cationic cotton, amphoteric cotton, sodiumacrylate-, acrylic acid-, acryInitrile- or acrylamides grafted cellosefiber and crosslinked fibers thereof, modified wool, modified silk,modified synthetic fiber, a saponified acylonitrile series of fiber,vinilon fiber partially esterfied by maleic acid, yarns made from one ormore of the foregoing fibers, and combinations thereof.
 2. The auxeticfiber according to claim 1 wherein the second component is straight whenin a dry state.
 3. The auxetic fiber according to claim 1 wherein thefirst component has a cross-sectional area that is between 0.001 and 1times the cross-sectional area of the second component.
 4. The auxeticfiber according to claim 1 wherein the first component comprisespolyvinyl alcohol.
 5. An array of auxetic fibers comprising: a first anda second auxetic fiber each comprising a first component, wherein thefirst component comprises a moisture activated shrinkable filament, andan elastomeric second component, and wherein the first component iswrapped about the second component in a helical configuration, andwherein the second component is selected from the group consisting of:siloxane, silicone rubber, natural rubber, poly(urethane) and itsderivatives, natural rubber, polyisoprene, bytyl rubber and itsderivatives, polybutadiene, styrene-butadiene rubber, chloroprenerubber, polychloroprene, neoprene, baypren, ethylene propylene rubber,ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylicrubber, fluorosilicone rubber, fluoroelastomers, perfluoroelastomers,polyether block amides, chlorosulfonated polyethylene, ethylene-vinylacetate, nitrile rubber or polyamides, nylon and combinations thereof.6. The array of claim 5 wherein the first and second auxetic fibers areof opposite handedness.
 7. The array of claim 5 wherein the secondcomponent comprises polyvinyl alcohol.
 8. The array of claim 5 whereinthe second component is straight when in a dry state.
 9. The array ofclaim 5 wherein the first auxetic fiber and the second auxetic fiberhave first component helices that are in phase.
 10. The array of claim 5comprising a woven structure.